2-axes roll types of crushers for crushing raw stones are known. One 2-axes roll type of crusher has one pair of rolls. The rolls have the respective teeth on the respective cylindrical surfaces thereof. Stones are crushed between the teeth of the roll on the one side and the teeth on the other side, both sets of teeth being continually oncoming during the rotation.
Also known are 4-axes roll types of crushers, each of which has two pairs of rolls. Stones are crushed by one pair of rolls to be of smaller size. Such crushed stones are further crushed by the other pair of rolls to be of still smaller size. Such a known crusher has only two crushing relations, those being the one crushing-relation of one pair of rolls and the other crushing-relation of the other pair of rolls.
Nowadays, roll types of crushers are frequently utilized at construction sites where materials such as concrete are crushed to be recycled. Such materials often include heterogeneous materials such as iron.
It is more desirable that there are crushing-relations among four rolls rather than two crushing-relations so that the crushing efficiency is higher. It is further desirable that foreign materials are smoothly removed without the machines being stopped.